It is often desirable to measure the "state of charge" of a battery. By performing such a measurement, one is able to determine the remaining lifetime of the battery. For example, a user of a portable electronic device such as a laptop computer may like to know when the battery powering the device is running low so that the battery can be recharged or replaced with a fully charged battery.
Measuring the state of charge of a battery is a surprisingly difficult thing to do. One reason for this is that a battery does not, in fact, store charge but rather stores chemical energy. The stored chemical energy is converted to electrical energy when a load is connected across the terminals of the battery. The efficiency at which a battery converts chemical energy to electrical energy is known as "conversion efficiency." In other words, the conversion efficiency of a battery indicates the percentage of stored chemical energy which is converted to electrical energy. Because of certain inefficiencies, batteries typically are not able to achieve a conversion efficiency of 100%. Moreover, conversion efficiency depends significantly on the temperature of the battery, the current drain presented by the load connected across the terminals, and the state of charge. Accordingly, a battery cannot in reality be regarded simply as a reservoir of charge.
Further complicating the measurement of state of charge, batteries discharge energy even in the absence of a load. The discharge of energy in the absence of a load is known as self-discharge. Depending upon the type of battery, the rate at which batteries self-discharge can vary from less than 1% per year to several percent per week. Accordingly, it is difficult to predict and to compensate for changes in charge resulting from self-discharge.
An example of a conventional state-of-charge indicator is a hydrometer used with a lead-acid car battery. At a given temperature, the concentration of the electrolyte in the battery is a good indicator of state of charge. The electrolyte concentration is directly related to the specific gravity of the electrolyte. The hydrometer measures the specific gravity. Temperature effects are taken into account by applying a correction factor, which needs to be looked up in a table. The use of a hydrometer is generally perceived to be too messy for use in consumer or portable products; accordingly this type of technique is not applicable in these areas.
A conventional noninvasive method for measuring state of charge of a battery is to measure the open-circuit voltage of the battery. A disadvantage of this approach is that it cannot be used when the device is running, since the measurement is made when the battery is off-load. In addition, the open-circuit voltage takes minutes or even hours to stabilize after a load has been disconnected. Moreover, the open-circuit voltage of many types of modern batteries is only loosely related to the state of charge, until just before the battery is completely discharged. Thus, it is difficult to obtain reasonable notice that the battery is running low using this technique.
Another method for measuring open-circuit voltage of a battery involves first stressing the battery with a high discharge for a short length of time using a special load in an attempt to overcome the stabilization time problem. However, this technique is not practical if battery lifetime is an issue.
Other methods for measuring state of charge include "charge accounting." Charge accounting is a technique based on the assumption that a battery conserves charge. Fuel gauges using charge-accounting techniques are widely used in equipment such as lap-top computers. However, imperfections of battery models resulting from variations in the manufacturing process do not allow fuel gauges to perform particularly well. To compensate for this inaccurate performance, batteries are sometimes assumed to be able to hold only 70% of their actual charge capacity in order to give a safety margin of 30%. Accordingly, a substantial portion of the charge capacity may not be utilized. In addition, variations in parameters from battery to battery, even variations in the same type of battery made by the same manufacturer, as well as battery aging, inhibit accurate measurement of charge by conventional fuel gauges using charge accounting.
In view of the foregoing, it is apparent that there is still a need for a storage device or battery that can be measured for state of charge easily and accurately.